1. Field of the Invention
The present invention relates generally to a closure for a container and particularly to a closure with means for indicating that a closure has been opened at least once.
2. Description of the Related Art
There is an increasing demand for tamper-indicating systems which ensure that a container is not re-filled with non-original contents. While it is relatively easy to produce some form of tamper-evidence, it is much more difficult to provide tamper-evidence which cannot be either overcome without causing the tamper-evidence system to activate, or activate and then return to a virtually visually identical state so as to appear non-activated.
A particularly useful method of providing tamper-evidence is to use a system in which a closure is initially located in a first position, but once removed can only be returned to a second position which is visually distinct from the first.
U.S. Pat. No. 5,738,231 describes a closure with a part which is moved during the opening process so that following opening it cannot pass back over projection on a container finish. The result is that the closure can only return to position which is axially displaced with respect to its original position.
WO 02/096771 describes a closure in which two parts are initially adjacent each other and during the opening process the structure of the closure is changed so that a gap is generated between the two parts as a visual indication that the closure has been opened at least once.
WO 2005/049443 and WO 2006/117505 also describe closures which generate a gap to indicate they have been opened at least once. In this case the gap is unobstructed. In other words, two parts of the closure are held apart without the requirement an obstruction.
What is not appreciated by the prior art is that such tamper-evident systems are only effective if they cannot be reversed. For example, in systems which use an obstructing member to hold two parts apart it is possible to cut the obstruction member to allow a gap to be closed. WO 2005/049443 and WO 2006/117505 describe closures which generate unobstructed gaps following relative rotation of one part with respect to another. The closures are provided with some internal mechanism for preventing the two parts from being rotated back to their original relative positions. For example, ratchet arrangements present on the side walls of the parts can be used to prevent unwanted rotation. Such “lateral” ratchet arrangements have been found to be defeatable if sufficient reverse turning torque is applied. Furthermore, it has been found that under some circumstances if significant axial force is applied to the closure the gap can be closed.
Accordingly, there is a need for an improved level of resistance to re-setting gap-generating closures to reduce the risk that the gap can be re-closed after a first opening.